Thermal-plume fibre optic tracking (T-POT) test for flow velocity measurement in groundwater boreholes

Abstract

Abstract. We develop an approach for measuring in-well fluid velocities using point electrical heating combined with spatially and temporally continuous temperature monitoring using distributed temperature sensing (DTS). The method uses a point heater to warm a discrete volume of water. The rate of advection of this plume, once the heating is stopped, equates to the average flow velocity in the well. We conducted thermal-plume fibre optic tracking (T-POT) tests in a borehole in a fractured rock aquifer with the heater at the same depth and multiple pumping rates. Tracking of the thermal plume peak allowed the spatially varying velocity to be estimated up to 50 m downstream from the heating point, depending on the pumping rate. The T-POT technique can be used to estimate the velocity throughout long intervals provided that thermal dilution due to inflows, dispersion, or cooling by conduction does not render the thermal pulse unresolvable with DTS. A complete flow log may be obtained by deploying the heater at multiple depths, or with multiple point heaters.

Distributed Temperature Sensing as a downhole tool in hydrogeology

“…In A‐DTS, the cable [e.g. Read et al ., ; Coleman et al ., ; Hausner et al ., ], borehole fluid [e.g., Leaf et al ., ; Read et al ., ], or surrounding rock formation is heated [ Bakker et al ., ]. The temperature response at some or all points during this process is then indicative of either the lithology, ambient groundwater flow, or in‐well flow, and depends closely on how the cable is deployed in the borehole and the nature of the thermal disturbance.…”

“…Electrical heating can be carried out along the entire or part of the length of the borehole or at point locations. Point heating can be achieved using small heating elements [ Sellwood et al ., ; Read et al ., ]. Uniform heating over lengths of borehole can be achieved with Joule heating along an electrical conductor [ Kurth et al ., ].…”

“…When this occurs such effects will need to be included in the interpretative framework of the A‐DTS data set. However, relatively simple calculations, for example using Rayleigh number analysis, can give already an indication of how important convection might be in a given system [ Read et al ., ]. Hence, an A‐DTS configuration might be designed to minimize the complications arising from such effects.…”

“…The test is carried out with a fiber optic cable installed in the borehole fluid. The point heating approach is possible with either the injection of fluid [ Leaf et al ., ], or with point electrical heating [ Sellwood et al ., , ; Read et al ., ]. For fluid injection, a thermally insulated hose is lowered down to the depth of interest and fluid pumped or allowed to drain in from the surface.…”

Groundwater Flow Quantification in Fractured Rock Boreholes Using Active Distributed Temperature Sensing Under Natural Gradient Conditions

“…Moreover, identification of active fractures under ambient flow conditions is much improved when the water column in the borehole is subjected to a rapid heating episode. DTS can also monitor heat movement and rate of heat dissipation due to vertical flow inside an open borehole, which can be used for locating transmissive fractures and for determining water flow rates in the borehole (Banks et al, 2014;Freifeld et al, 2008;Hurtig et al, 1994;Leaf et al, 2012;Read et al, 2015Read et al, , 2014Sakaguchi & Matsushima, 2000;Sellwood et al, 2015bSellwood et al, , 2015aYamano & Goto, 2005). However, Pehme et al (2010) clearly showed that open-hole temperature profiles can mask many active fractures and it is well-known that hydraulic cross connection can alter the natural flow conditions of an aquifer.…”